Breaking Biology

[15102013] Evolution, Natural Selection & All That Good Stuff ≡

Posted on Monday, October 21, 2013 at 7:11 PM

Evolution or Organismic Evolution is about changes in populations, species or groups of species. Evolution occurs because populations vary by the frequency of heritable traits that appear from one generation to the next. These traits are represented by alleles for genes that modify morphology (form or structure), physiology or behavior. Thus, evolution is changes in allele frequencies in populations over time.

Microevolution describes the details of how populations of organisms change from generation to generation and how new species originate.

Evidence for evolution is provided by the following five scientific disciplines:

1. Paleontology provides fossils that reveal the prehistoric existence of extinct species. As a result, changes in species and the formation of new species can be studied.

Fossil deposits are often found among sediment layers, where the deepest fossils represent the oldest specimens. For example, fossil oysters removed from successive layers of sediment show gradual changes in the size of the oyster shell alternating with rapid changes in shell size. Large, rapid changes produced new species.

2. Biogeography uses geography to describe the distribution of species. This information has revealed that unrelated species in different regions of the world look alike when found in similar environments. This provides strong evidence for the role of natural selection in evolution.

Rabbits did not exist in Australia until introduced by humans. A native Australian hare wallaby resembles a rabbit both in structure and habit. As similar as these two animals appear, they are not that closely related.The rabbit is a placental mammal, while the wallaby is a marsupial mammal. The fetus of a placental mammal develops in the female uterus, obtaining nourishment from the mother through the placenta. The fetus of a marsupial leaves the mother’s uterus at an early stage of development and completes the remaining development while attached to a teat in the abdominal pouch. The great similarity of the rabbit and the wallaby is the result of natural selection.

3. Embryology reveals similar stages in development ( ontogeny) among related species. The similarities help establish evolutionary relationships ( phylogeny).

Gill slits and tails are found in fish, chicken, pig, and human embryos.

4. Comparative anatomy describes two kinds of structures that contribute to the identification of evolutionary relationships among species.

Homologous structures are body parts that resemble one another in different species because they have evolved from a common ancestor. Because anatomy may be modified for survival in specific environments, homologous structures may look different, but will resemble one another in pattern (how they are put together). The forelimbs of cats, bats, whales, and humans are homologous because they have all evolved from a common ancestral mammal.
Analogous structures are body parts that resemble one another in different species, not because they have evolved from a common ancestor, but because they evolved independently as adaptations to their environments. The fins and body shapes of sharks, penguins, and porpoises are analogous because they are adaptations to swimming.

5. Molecular biology examines the nucleotide and amino acid sequences of DNA and proteins from different species. Closely related species share higher percentages of sequences than species distantly related. In addition, all living things share the same genetic code. This data strongly favors evolution of different species through modification of ancestral genetic information. More than 98% of the nucleotide sequences in humans and chimpanzees are identical.

Natural selection is the differences in survival and reproduction among individuals in a population as a result of their interaction with the environment. In other words, some individuals possess alleles (genotypes) that generate traits (phenotypes) that enable them to cope more successfully in their environment than other individuals. The more successful individuals produce more offspring. Superior inherited traits are adaptations to the environment and increase an

individual’s fitness, or relative ability to survive and leave offspring. When the environment favors a trait, that is, when a trait increases the survival of its bearer, selection is said to act for that trait. In contrast, selection is said to act against unfavorable traits. Favorable traits are adaptive, while unfavorable traits are maladaptive.

Darwin presented his theory for evolution by natural selection using the following arguments:

1. Populations possess an enormous reproductive potential. Darwin calculated that two elephants would produce a population of 19 million individuals after 750 years if all offspring survived to reproductive maturity and fostered their normal number of offspring.

2. Population sizes remain stable. Darwin observed that populations generally fluctuate around a constant size.

3. Resources are limited. Resources, such as food, water, or light, do not increase as populations grow larger.

4. Individuals compete for survival. Eventually, the needs of a growing population will exceed the available resources. As a result, individuals must compete for resources.

5. There is variation among individuals in a population. Most traits reveal considerable variety in their form. In humans, for example, skin, hair, and eye color occur as continuous variation from very dark to very light.

6. Much variation is heritable. Most traits are produced by the action of enzymes that are coded by DNA. DNA is the hereditary information that is passed from generation to generation. This contrasts with characteristics acquired during the life of an organism. The amputation of a limb, for example, is not heritable.

7. Only the most fit individuals survive. “Survival of the fittest” occurs because individuals with traits best adapted for survival and reproduction are able to outcompete other individuals for resources and mates.

8. Evolution occurs as favorable traits accumulate in the population. The best adapted individuals survive and leave offspring who inherit the traits of their parents. In turn, the best adapted of these offspring leave the most

offspring. Over time, traits best adapted for survival and reproduction

Hardy-Weinberg Condition:

1. No mutation

2. No selection

3. No gene flow (no migration)

4. Large populations (no genetic drift)

5. Random mating

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